content: /*
   * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
   *
   *
   *
   *
   *
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   */
  
  /*
   *
   *
   *
   *
   *
   * Written by Doug Lea and Martin Buchholz with assistance from members of
   * JCP JSR-166 Expert Group and released to the public domain, as explained
   * at http://creativecommons.org/publicdomain/zero/1.0/
   */
  
  package io.akarin.api.internal.utils;
  
  import java.util.AbstractQueue;
  import java.util.ArrayList;
  import java.util.Collection;
  import java.util.Iterator;
  import java.util.NoSuchElementException;
  import java.util.Queue;
  import java.util.Spliterator;
  import java.util.Spliterators;
  import java.util.function.Consumer;
  
  import io.akarin.api.internal.Akari;
  
  /**
   * An unbounded thread-safe {@linkplain Queue queue} based on linked nodes.
   * This queue orders elements FIFO (first-in-first-out).
   * The <em>head</em> of the queue is that element that has been on the
   * queue the longest time.
   * The <em>tail</em> of the queue is that element that has been on the
   * queue the shortest time. New elements
   * are inserted at the tail of the queue, and the queue retrieval
   * operations obtain elements at the head of the queue.
   * A {@code ConcurrentLinkedQueue} is an appropriate choice when
   * many threads will share access to a common collection.
   * Like most other concurrent collection implementations, this class
   * does not permit the use of {@code null} elements.
   *
   * <p>This implementation employs an efficient <em>non-blocking</em>
   * algorithm based on one described in <a
   * href="http://www.cs.rochester.edu/u/michael/PODC96.html"> Simple,
   * Fast, and Practical Non-Blocking and Blocking Concurrent Queue
   * Algorithms</a> by Maged M. Michael and Michael L. Scott.
   *
   * <p>Iterators are <i>weakly consistent</i>, returning elements
   * reflecting the state of the queue at some point at or since the
   * creation of the iterator.  They do <em>not</em> throw {@link
   * java.util.ConcurrentModificationException}, and may proceed concurrently
   * with other operations.  Elements contained in the queue since the creation
   * of the iterator will be returned exactly once.
   *
   * <p>Beware that, unlike in most collections, the {@code size} method
   * is <em>NOT</em> a constant-time operation. Because of the
   * asynchronous nature of these queues, determining the current number
   * of elements requires a traversal of the elements, and so may report
   * inaccurate results if this collection is modified during traversal.
   * Additionally, the bulk operations {@code addAll},
   * {@code removeAll}, {@code retainAll}, {@code containsAll},
   * {@code equals}, and {@code toArray} are <em>not</em> guaranteed
   * to be performed atomically. For example, an iterator operating
   * concurrently with an {@code addAll} operation might view only some
   * of the added elements.
   *
   * <p>This class and its iterator implement all of the <em>optional</em>
   * methods of the {@link Queue} and {@link Iterator} interfaces.
   *
   * <p>Memory consistency effects: As with other concurrent
   * collections, actions in a thread prior to placing an object into a
   * {@code ConcurrentLinkedQueue}
   * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
   * actions subsequent to the access or removal of that element from
   * the {@code ConcurrentLinkedQueue} in another thread.
   *
   * <p>This class is a member of the
   * <a href="{@docRoot}/../technotes/guides/collections/index.html">
   * Java Collections Framework</a>.
   *
   * @since 1.5
   * @author Doug Lea
   * @param <E> the type of elements held in this collection
   */
  public class CheckedConcurrentLinkedQueue<E> extends AbstractQueue<E>
          implements Queue<E>, java.io.Serializable {
      private static final long serialVersionUID = 196745693267521676L;
      
      public E poll(com.google.common.base.Predicate<E> predicate, E singalInstance) {
          restartFromHead:
          for (;;) {
              for (Node<E> h = head, p = h, q;;) {
                  E item = p.item;
                  
                  if (predicate.apply(item)) return singalInstance; // Test predicate
  
                  if (item != null && p.casItem(item, null)) {
                      // Successful CAS is the linearization point
                      // for item to be removed from this queue.
                      if (p != h) // hop two nodes at a time
                          updateHead(h, ((q = p.next) != null) ? q : p);
                      return item;
                  }
                  else if ((q = p.next) == null) {
                      updateHead(h, p);
                      return null;
                  }
                  else if (p == q)
                      continue restartFromHead;
                  else
                      p = q;
              }
          }
      }
  
      /*
       * This is a modification of the Michael & Scott algorithm,
       * adapted for a garbage-collected environment, with support for
       * interior node deletion (to support remove(Object)).  For
       * explanation, read the paper.
       *
       * Note that like most non-blocking algorithms in this package,
       * this implementation relies on the fact that in garbage
       * collected systems, there is no possibility of ABA problems due
       * to recycled nodes, so there is no need to use "counted
       * pointers" or related techniques seen in versions used in
       * non-GC'ed settings.
       *
       * The fundamental invariants are:
       * - There is exactly one (last) Node with a null next reference,
       *   which is CASed when enqueueing.  This last Node can be
       *   reached in O(1) time from tail, but tail is merely an
       *   optimization - it can always be reached in O(N) time from
       *   head as well.
       * - The elements contained in the queue are the non-null items in
       *   Nodes that are reachable from head.  CASing the item
       *   reference of a Node to null atomically removes it from the
       *   queue.  Reachability of all elements from head must remain
       *   true even in the case of concurrent modifications that cause
       *   head to advance.  A dequeued Node may remain in use
       *   indefinitely due to creation of an Iterator or simply a
       *   poll() that has lost its time slice.
       *
       * The above might appear to imply that all Nodes are GC-reachable
       * from a predecessor dequeued Node.  That would cause two problems:
       * - allow a rogue Iterator to cause unbounded memory retention
       * - cause cross-generational linking of old Nodes to new Nodes if
       *   a Node was tenured while live, which generational GCs have a
       *   hard time dealing with, causing repeated major collections.
       * However, only non-deleted Nodes need to be reachable from
       * dequeued Nodes, and reachability does not necessarily have to
       * be of the kind understood by the GC.  We use the trick of
       * linking a Node that has just been dequeued to itself.  Such a
       * self-link implicitly means to advance to head.
       *
       * Both head and tail are permitted to lag.  In fact, failing to
       * update them every time one could is a significant optimization
       * (fewer CASes). As with LinkedTransferQueue (see the internal
       * documentation for that class), we use a slack threshold of two;
       * that is, we update head/tail when the current pointer appears
       * to be two or more steps away from the first/last node.
       *
       * Since head and tail are updated concurrently and independently,
       * it is possible for tail to lag behind head (why not)?
       *
       * CASing a Node's item reference to null atomically removes the
       * element from the queue.  Iterators skip over Nodes with null
       * items.  Prior implementations of this class had a race between
       * poll() and remove(Object) where the same element would appear
       * to be successfully removed by two concurrent operations.  The
       * method remove(Object) also lazily unlinks deleted Nodes, but
       * this is merely an optimization.
       *
       * When constructing a Node (before enqueuing it) we avoid paying
       * for a volatile write to item by using Unsafe.putObject instead
       * of a normal write.  This allows the cost of enqueue to be
       * "one-and-a-half" CASes.
       *
       * Both head and tail may or may not point to a Node with a
       * non-null item.  If the queue is empty, all items must of course
       * be null.  Upon creation, both head and tail refer to a dummy
       * Node with null item.  Both head and tail are only updated using
       * CAS, so they never regress, although again this is merely an
       * optimization.
       */
  
      private static class Node<E> {
          volatile E item;
          volatile Node<E> next;
  
          /**
           * Constructs a new node.  Uses relaxed write because item can
           * only be seen after publication via casNext.
           */
          Node(E item) {
              UNSAFE.putObject(this, itemOffset, item);
          }
  
          boolean casItem(E cmp, E val) {
              return UNSAFE.compareAndSwapObject(this, itemOffset, cmp, val);
          }
  
          void lazySetNext(Node<E> val) {
              UNSAFE.putOrderedObject(this, nextOffset, val);
          }
  
          boolean casNext(Node<E> cmp, Node<E> val) {
              return UNSAFE.compareAndSwapObject(this, nextOffset, cmp, val);
          }
  
          // Unsafe mechanics
  
          private static final sun.misc.Unsafe UNSAFE;
          private static final long itemOffset;
          private static final long nextOffset;
  
          static {
              try {
                  UNSAFE = Akari.UNSAFE;
                  Class<?> k = Node.class;
                  itemOffset = UNSAFE.objectFieldOffset
                      (k.getDeclaredField("item"));
                  nextOffset = UNSAFE.objectFieldOffset
                      (k.getDeclaredField("next"));
              } catch (Exception e) {
                  throw new Error(e);
              }
          }
      }
  
      /**
       * A node from which the first live (non-deleted) node (if any)
       * can be reached in O(1) time.
       * Invariants:
       * - all live nodes are reachable from head via succ()
       * - head != null
       * - (tmp = head).next != tmp || tmp != head
       * Non-invariants:
       * - head.item may or may not be null.
       * - it is permitted for tail to lag behind head, that is, for tail
       *   to not be reachable from head!
       */
      private transient volatile Node<E> head;
  
      /**
       * A node from which the last node on list (that is, the unique
       * node with node.next == null) can be reached in O(1) time.
       * Invariants:
       * - the last node is always reachable from tail via succ()
       * - tail != null
       * Non-invariants:
       * - tail.item may or may not be null.
       * - it is permitted for tail to lag behind head, that is, for tail
       *   to not be reachable from head!
       * - tail.next may or may not be self-pointing to tail.
       */
      private transient volatile Node<E> tail;
  
      /**
       * Creates a {@code ConcurrentLinkedQueue} that is initially empty.
       */
      public CheckedConcurrentLinkedQueue() {
          head = tail = new Node<E>(null);
      }
  
      /**
       * Creates a {@code ConcurrentLinkedQueue}
       * initially containing the elements of the given collection,
       * added in traversal order of the collection's iterator.
       *
       * @param c the collection of elements to initially contain
       * @throws NullPointerException if the specified collection or any
       *         of its elements are null
       */
      public CheckedConcurrentLinkedQueue(Collection<? extends E> c) {
          Node<E> h = null, t = null;
          for (E e : c) {
              checkNotNull(e);
              Node<E> newNode = new Node<E>(e);
              if (h == null)
                  h = t = newNode;
              else {
                  t.lazySetNext(newNode);
                  t = newNode;
              }
          }
          if (h == null)
              h = t = new Node<E>(null);
          head = h;
          tail = t;
      }
  
      // Have to override just to update the javadoc
  
      /**
       * Inserts the specified element at the tail of this queue.
       * As the queue is unbounded, this method will never throw
       * {@link IllegalStateException} or return {@code false}.
       *
       * @return {@code true} (as specified by {@link Collection#add})
       * @throws NullPointerException if the specified element is null
       */
      @Override
      public boolean add(E e) {
          return offer(e);
      }
  
      /**
       * Tries to CAS head to p. If successful, repoint old head to itself
       * as sentinel for succ(), below.
       */
      final void updateHead(Node<E> h, Node<E> p) {
          if (h != p && casHead(h, p))
              h.lazySetNext(h);
      }
  
      /**
       * Returns the successor of p, or the head node if p.next has been
       * linked to self, which will only be true if traversing with a
       * stale pointer that is now off the list.
       */
      final Node<E> succ(Node<E> p) {
          Node<E> next = p.next;
          return (p == next) ? head : next;
      }
  
      /**
       * Inserts the specified element at the tail of this queue.
       * As the queue is unbounded, this method will never return {@code false}.
       *
       * @return {@code true} (as specified by {@link Queue#offer})
       * @throws NullPointerException if the specified element is null
       */
      @Override
      public boolean offer(E e) {
          checkNotNull(e);
          final Node<E> newNode = new Node<E>(e);
  
          for (Node<E> t = tail, p = t;;) {
              Node<E> q = p.next;
              if (q == null) {
                  // p is last node
                  if (p.casNext(null, newNode)) {
                      // Successful CAS is the linearization point
                      // for e to become an element of this queue,
                      // and for newNode to become "live".
                      if (p != t) // hop two nodes at a time
                          casTail(t, newNode);  // Failure is OK.
                      return true;
                  }
                  // Lost CAS race to another thread; re-read next
              }
              else if (p == q)
                  // We have fallen off list.  If tail is unchanged, it
                  // will also be off-list, in which case we need to
                  // jump to head, from which all live nodes are always
                  // reachable.  Else the new tail is a better bet.
                  p = (t != (t = tail)) ? t : head;
              else
                  // Check for tail updates after two hops.
                  p = (p != t && t != (t = tail)) ? t : q;
          }
      }
  
      @Override
      public E poll() {
          restartFromHead:
          for (;;) {
              for (Node<E> h = head, p = h, q;;) {
                  E item = p.item;
  
                  if (item != null && p.casItem(item, null)) {
                      // Successful CAS is the linearization point
                      // for item to be removed from this queue.
                      if (p != h) // hop two nodes at a time
                          updateHead(h, ((q = p.next) != null) ? q : p);
                      return item;
                  }
                  else if ((q = p.next) == null) {
                      updateHead(h, p);
                      return null;
                  }
                  else if (p == q)
                      continue restartFromHead;
                  else
                      p = q;
              }
          }
      }
  
      @Override
      public E peek() {
          restartFromHead:
          for (;;) {
              for (Node<E> h = head, p = h, q;;) {
                  E item = p.item;
                  if (item != null || (q = p.next) == null) {
                      updateHead(h, p);
                      return item;
                  }
                  else if (p == q)
                      continue restartFromHead;
                  else
                      p = q;
              }
          }
      }
  
      /**
       * Returns the first live (non-deleted) node on list, or null if none.
       * This is yet another variant of poll/peek; here returning the
       * first node, not element.  We could make peek() a wrapper around
       * first(), but that would cost an extra volatile read of item,
       * and the need to add a retry loop to deal with the possibility
       * of losing a race to a concurrent poll().
       */
      Node<E> first() {
          restartFromHead:
          for (;;) {
              for (Node<E> h = head, p = h, q;;) {
                  boolean hasItem = (p.item != null);
                  if (hasItem || (q = p.next) == null) {
                      updateHead(h, p);
                      return hasItem ? p : null;
                  }
                  else if (p == q)
                      continue restartFromHead;
                  else
                      p = q;
              }
          }
      }
  
      /**
       * Returns {@code true} if this queue contains no elements.
       *
       * @return {@code true} if this queue contains no elements
       */
      @Override
      public boolean isEmpty() {
          return first() == null;
      }
  
      /**
       * Returns the number of elements in this queue.  If this queue
       * contains more than {@code Integer.MAX_VALUE} elements, returns
       * {@code Integer.MAX_VALUE}.
       *
       * <p>Beware that, unlike in most collections, this method is
       * <em>NOT</em> a constant-time operation. Because of the
       * asynchronous nature of these queues, determining the current
       * number of elements requires an O(n) traversal.
       * Additionally, if elements are added or removed during execution
       * of this method, the returned result may be inaccurate.  Thus,
       * this method is typically not very useful in concurrent
       * applications.
       *
       * @return the number of elements in this queue
       */
      @Override
      public int size() {
          int count = 0;
          for (Node<E> p = first(); p != null; p = succ(p))
              if (p.item != null)
                  // Collection.size() spec says to max out
                  if (++count == Integer.MAX_VALUE)
                      break;
          return count;
      }
  
      /**
       * Returns {@code true} if this queue contains the specified element.
       * More formally, returns {@code true} if and only if this queue contains
       * at least one element {@code e} such that {@code o.equals(e)}.
       *
       * @param o object to be checked for containment in this queue
       * @return {@code true} if this queue contains the specified element
       */
      @Override
      public boolean contains(Object o) {
          if (o == null) return false;
          for (Node<E> p = first(); p != null; p = succ(p)) {
              E item = p.item;
              if (item != null && o.equals(item))
                  return true;
          }
          return false;
      }
  
      /**
       * Removes a single instance of the specified element from this queue,
       * if it is present.  More formally, removes an element {@code e} such
       * that {@code o.equals(e)}, if this queue contains one or more such
       * elements.
       * Returns {@code true} if this queue contained the specified element
       * (or equivalently, if this queue changed as a result of the call).
       *
       * @param o element to be removed from this queue, if present
       * @return {@code true} if this queue changed as a result of the call
       */
      @Override
      public boolean remove(Object o) {
          if (o != null) {
              Node<E> next, pred = null;
              for (Node<E> p = first(); p != null; pred = p, p = next) {
                  boolean removed = false;
                  E item = p.item;
                  if (item != null) {
                      if (!o.equals(item)) {
                          next = succ(p);
                          continue;
                      }
                      removed = p.casItem(item, null);
                  }
  
                  next = succ(p);
                  if (pred != null && next != null) // unlink
                      pred.casNext(p, next);
                  if (removed)
                      return true;
              }
          }
          return false;
      }
  
      /**
       * Appends all of the elements in the specified collection to the end of
       * this queue, in the order that they are returned by the specified
       * collection's iterator.  Attempts to {@code addAll} of a queue to
       * itself result in {@code IllegalArgumentException}.
       *
       * @param c the elements to be inserted into this queue
       * @return {@code true} if this queue changed as a result of the call
       * @throws NullPointerException if the specified collection or any
       *         of its elements are null
       * @throws IllegalArgumentException if the collection is this queue
       */
      @Override
      public boolean addAll(Collection<? extends E> c) {
          if (c == this)
              // As historically specified in AbstractQueue#addAll
              throw new IllegalArgumentException();
  
          // Copy c into a private chain of Nodes
          Node<E> beginningOfTheEnd = null, last = null;
          for (E e : c) {
              checkNotNull(e);
              Node<E> newNode = new Node<E>(e);
              if (beginningOfTheEnd == null)
                  beginningOfTheEnd = last = newNode;
              else {
                  last.lazySetNext(newNode);
                  last = newNode;
              }
          }
          if (beginningOfTheEnd == null)
              return false;
  
          // Atomically append the chain at the tail of this collection
          for (Node<E> t = tail, p = t;;) {
              Node<E> q = p.next;
              if (q == null) {
                  // p is last node
                  if (p.casNext(null, beginningOfTheEnd)) {
                      // Successful CAS is the linearization point
                      // for all elements to be added to this queue.
                      if (!casTail(t, last)) {
                          // Try a little harder to update tail,
                          // since we may be adding many elements.
                          t = tail;
                          if (last.next == null)
                              casTail(t, last);
                      }
                      return true;
                  }
                  // Lost CAS race to another thread; re-read next
              }
              else if (p == q)
                  // We have fallen off list.  If tail is unchanged, it
                  // will also be off-list, in which case we need to
                  // jump to head, from which all live nodes are always
                  // reachable.  Else the new tail is a better bet.
                  p = (t != (t = tail)) ? t : head;
              else
                  // Check for tail updates after two hops.
                  p = (p != t && t != (t = tail)) ? t : q;
          }
      }
  
      /**
       * Returns an array containing all of the elements in this queue, in
       * proper sequence.
       *
       * <p>The returned array will be "safe" in that no references to it are
       * maintained by this queue.  (In other words, this method must allocate
       * a new array).  The caller is thus free to modify the returned array.
       *
       * <p>This method acts as bridge between array-based and collection-based
       * APIs.
       *
       * @return an array containing all of the elements in this queue
       */
      @Override
      public Object[] toArray() {
          // Use ArrayList to deal with resizing.
          ArrayList<E> al = new ArrayList<E>();
          for (Node<E> p = first(); p != null; p = succ(p)) {
              E item = p.item;
              if (item != null)
                  al.add(item);
          }
          return al.toArray();
      }
  
      /**
       * Returns an array containing all of the elements in this queue, in
       * proper sequence; the runtime type of the returned array is that of
       * the specified array.  If the queue fits in the specified array, it
       * is returned therein.  Otherwise, a new array is allocated with the
       * runtime type of the specified array and the size of this queue.
       *
       * <p>If this queue fits in the specified array with room to spare
       * (i.e., the array has more elements than this queue), the element in
       * the array immediately following the end of the queue is set to
       * {@code null}.
       *
       * <p>Like the {@link #toArray()} method, this method acts as bridge between
       * array-based and collection-based APIs.  Further, this method allows
       * precise control over the runtime type of the output array, and may,
       * under certain circumstances, be used to save allocation costs.
       *
       * <p>Suppose {@code x} is a queue known to contain only strings.
       * The following code can be used to dump the queue into a newly
       * allocated array of {@code String}:
       *
       *  <pre> {@code String[] y = x.toArray(new String[0]);}</pre>
       *
       * Note that {@code toArray(new Object[0])} is identical in function to
       * {@code toArray()}.
       *
       * @param a the array into which the elements of the queue are to
       *          be stored, if it is big enough; otherwise, a new array of the
       *          same runtime type is allocated for this purpose
       * @return an array containing all of the elements in this queue
       * @throws ArrayStoreException if the runtime type of the specified array
       *         is not a supertype of the runtime type of every element in
       *         this queue
       * @throws NullPointerException if the specified array is null
       */
      @Override
      @SuppressWarnings("unchecked")
      public <T> T[] toArray(T[] a) {
          // try to use sent-in array
          int k = 0;
          Node<E> p;
          for (p = first(); p != null && k < a.length; p = succ(p)) {
              E item = p.item;
              if (item != null)
                  a[k++] = (T)item;
          }
          if (p == null) {
              if (k < a.length)
                  a[k] = null;
              return a;
          }
  
          // If won't fit, use ArrayList version
          ArrayList<E> al = new ArrayList<E>();
          for (Node<E> q = first(); q != null; q = succ(q)) {
              E item = q.item;
              if (item != null)
                  al.add(item);
          }
          return al.toArray(a);
      }
  
      /**
       * Returns an iterator over the elements in this queue in proper sequence.
       * The elements will be returned in order from first (head) to last (tail).
       *
       * <p>The returned iterator is
       * <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>.
       *
       * @return an iterator over the elements in this queue in proper sequence
       */
      @Override
      public Iterator<E> iterator() {
          return new Itr();
      }
  
      private class Itr implements Iterator<E> {
          /**
           * Next node to return item for.
           */
          private Node<E> nextNode;
  
          /**
           * nextItem holds on to item fields because once we claim
           * that an element exists in hasNext(), we must return it in
           * the following next() call even if it was in the process of
           * being removed when hasNext() was called.
           */
          private E nextItem;
  
          /**
           * Node of the last returned item, to support remove.
           */
          private Node<E> lastRet;
  
          Itr() {
              advance();
          }
  
          /**
           * Moves to next valid node and returns item to return for
           * next(), or null if no such.
           */
          private E advance() {
              lastRet = nextNode;
              E x = nextItem;
  
              Node<E> pred, p;
              if (nextNode == null) {
                  p = first();
                  pred = null;
              } else {
                  pred = nextNode;
                  p = succ(nextNode);
              }
  
              for (;;) {
                  if (p == null) {
                      nextNode = null;
                      nextItem = null;
                      return x;
                  }
                  E item = p.item;
                  if (item != null) {
                      nextNode = p;
                      nextItem = item;
                      return x;
                  } else {
                      // skip over nulls
                      Node<E> next = succ(p);
                      if (pred != null && next != null)
                          pred.casNext(p, next);
                      p = next;
                  }
              }
          }
  
          @Override
          public boolean hasNext() {
              return nextNode != null;
          }
  
          @Override
          public E next() {
              if (nextNode == null) throw new NoSuchElementException();
              return advance();
          }
  
          @Override
          public void remove() {
              Node<E> l = lastRet;
              if (l == null) throw new IllegalStateException();
              // rely on a future traversal to relink.
              l.item = null;
              lastRet = null;
          }
      }
  
      /**
       * Saves this queue to a stream (that is, serializes it).
       *
       * @param s the stream
       * @throws java.io.IOException if an I/O error occurs
       * @serialData All of the elements (each an {@code E}) in
       * the proper order, followed by a null
       */
      private void writeObject(java.io.ObjectOutputStream s)
          throws java.io.IOException {
  
          // Write out any hidden stuff
          s.defaultWriteObject();
  
          // Write out all elements in the proper order.
          for (Node<E> p = first(); p != null; p = succ(p)) {
              Object item = p.item;
              if (item != null)
                  s.writeObject(item);
          }
  
          // Use trailing null as sentinel
          s.writeObject(null);
      }
  
      /**
       * Reconstitutes this queue from a stream (that is, deserializes it).
       * @param s the stream
       * @throws ClassNotFoundException if the class of a serialized object
       *         could not be found
       * @throws java.io.IOException if an I/O error occurs
       */
      private void readObject(java.io.ObjectInputStream s)
          throws java.io.IOException, ClassNotFoundException {
          s.defaultReadObject();
  
          // Read in elements until trailing null sentinel found
          Node<E> h = null, t = null;
          Object item;
          while ((item = s.readObject()) != null) {
              @SuppressWarnings("unchecked")
              Node<E> newNode = new Node<E>((E) item);
              if (h == null)
                  h = t = newNode;
              else {
                  t.lazySetNext(newNode);
                  t = newNode;
              }
          }
          if (h == null)
              h = t = new Node<E>(null);
          head = h;
          tail = t;
      }
  
      /** A customized variant of Spliterators.IteratorSpliterator */
      static final class CLQSpliterator<E> implements Spliterator<E> {
          static final int MAX_BATCH = 1 << 25;  // max batch array size;
          final CheckedConcurrentLinkedQueue<E> queue;
          Node<E> current;    // current node; null until initialized
          int batch;          // batch size for splits
          boolean exhausted;  // true when no more nodes
          CLQSpliterator(CheckedConcurrentLinkedQueue<E> queue) {
              this.queue = queue;
          }
  
          @Override
          public Spliterator<E> trySplit() {
              Node<E> p;
              final CheckedConcurrentLinkedQueue<E> q = this.queue;
              int b = batch;
              int n = (b <= 0) ? 1 : (b >= MAX_BATCH) ? MAX_BATCH : b + 1;
              if (!exhausted &&
                  ((p = current) != null || (p = q.first()) != null) &&
                  p.next != null) {
                  Object[] a = new Object[n];
                  int i = 0;
                  do {
                      if ((a[i] = p.item) != null)
                          ++i;
                      if (p == (p = p.next))
                          p = q.first();
                  } while (p != null && i < n);
                  if ((current = p) == null)
                      exhausted = true;
                  if (i > 0) {
                      batch = i;
                      return Spliterators.spliterator
                          (a, 0, i, Spliterator.ORDERED | Spliterator.NONNULL |
                           Spliterator.CONCURRENT);
                  }
              }
              return null;
          }
  
          @Override
          public void forEachRemaining(Consumer<? super E> action) {
              Node<E> p;
              if (action == null) throw new NullPointerException();
              final CheckedConcurrentLinkedQueue<E> q = this.queue;
              if (!exhausted &&
                  ((p = current) != null || (p = q.first()) != null)) {
                  exhausted = true;
                  do {
                      E e = p.item;
                      if (p == (p = p.next))
                          p = q.first();
                      if (e != null)
                          action.accept(e);
                  } while (p != null);
              }
          }
  
          @Override
          public boolean tryAdvance(Consumer<? super E> action) {
              Node<E> p;
              if (action == null) throw new NullPointerException();
              final CheckedConcurrentLinkedQueue<E> q = this.queue;
              if (!exhausted &&
                  ((p = current) != null || (p = q.first()) != null)) {
                  E e;
                  do {
                      e = p.item;
                      if (p == (p = p.next))
                          p = q.first();
                  } while (e == null && p != null);
                  if ((current = p) == null)
                      exhausted = true;
                  if (e != null) {
                      action.accept(e);
                      return true;
                  }
              }
              return false;
          }
  
          @Override
          public long estimateSize() { return Long.MAX_VALUE; }
  
          @Override
          public int characteristics() {
              return Spliterator.ORDERED | Spliterator.NONNULL |
                  Spliterator.CONCURRENT;
          }
      }
  
      /**
       * Returns a {@link Spliterator} over the elements in this queue.
       *
       * <p>The returned spliterator is
       * <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>.
       *
       * <p>The {@code Spliterator} reports {@link Spliterator#CONCURRENT},
       * {@link Spliterator#ORDERED}, and {@link Spliterator#NONNULL}.
       *
       * @implNote
       * The {@code Spliterator} implements {@code trySplit} to permit limited
       * parallelism.
       *
       * @return a {@code Spliterator} over the elements in this queue
       * @since 1.8
       */
      @Override
      public Spliterator<E> spliterator() {
          return new CLQSpliterator<E>(this);
      }
  
      /**
       * Throws NullPointerException if argument is null.
       *
       * @param v the element
       */
      private static void checkNotNull(Object v) {
          if (v == null)
              throw new NullPointerException();
      }
  
      private boolean casTail(Node<E> cmp, Node<E> val) {
          return UNSAFE.compareAndSwapObject(this, tailOffset, cmp, val);
      }
  
      private boolean casHead(Node<E> cmp, Node<E> val) {
          return UNSAFE.compareAndSwapObject(this, headOffset, cmp, val);
      }
  
      // Unsafe mechanics
  
      private static final sun.misc.Unsafe UNSAFE;
      private static final long headOffset;
      private static final long tailOffset;
      static {
          try {
              UNSAFE = Akari.UNSAFE;
              Class<?> k = CheckedConcurrentLinkedQueue.class;
              headOffset = UNSAFE.objectFieldOffset
                  (k.getDeclaredField("head"));
              tailOffset = UNSAFE.objectFieldOffset
                  (k.getDeclaredField("tail"));
          } catch (Exception e) {
              throw new Error(e);
          }
      }
  }
  